(1) Field of the Invention
The present invention relates to a method of manufacturing insulators having excellent high-voltage performance such as polymer insulators or polymer hollow insulators used in an open-air space, especially relates to a method of manufacturing insulators which prevent a generation of electric defects such as voids at a boundary between a sheath arranged on an outer surface of a core member and a shed arranged on an outer surface of the sheath.
(2) Description of Related Arts
As insulators having an excellent high-voltage performance, polymer insulators using a solid core member and polymer hollow insulators using a hollow core member are known. These insulators have a construction such that an outer surface of the core member is covered with the sheath made of polymer materials and a plurality of sheds made of polymer materials are arranged equidistantly on an outer surface of the sheath.
The shed prevents generation of flashovers on an outer surface of the sheath. The flashover is caused by electric water passage generated on an outer surface of the sheath due to rain,for example. Therefore, the shed and the sheath must be connected tightly so as not to generate defects such as voids therebetween. This is because, if the defects such as voids are generated at the boundary between the sheath and the shed, a dielectric strength of the insulator is decreased due to the electric water passage. Particularly, a dielectric fracture is liable to be generated at an inner portion of the shed. Moreover, corona discharge due to the void causes radio wave interference, and thus voids are not preferred.
It is understood that a reason for void generation at the boundary mentioned above is as follows. When the sheath and the shed are connected by curing, a water component or a gas component included in non-cured polymer materials is closed in the boundary. When they are connected by adhesives, a water component or the like included in the adhesives is closed in the boundary.
Japanese Patent Laid-Open Publication No. 61-181015 discloses a method of connecting the sheath and the shed. In this publication, there is disclosed (1) a method of connecting the sheath made of non-cured polymer materials and the shed made of cured polymer materials by curing the sheath, and (2) a method of connecting the sheath made of cured polymer materials and the shed made of cured polymer materials by using adhesives in such a manner that the shed is adhered to a recess arranged on an outer surface of the sheath.
In the method (1) in which the connection is performed by curing the sheath, since a securing force F at the connecting portion 13' is uniform along an inner surface of the shed 5' as shown in FIG. 6, a rubber flow 12' of the sheath 3? at the connecting portion 13' is moved inwardly. Therefore, a water component or a gas component included in the non-cured polymer materials of the sheath 3', which are moved together with the rubber flow 12', is flowed also inwardly, so that it is closed in the connecting portion 13' or in the sheath 3' close to the connecting portion 13'. As a result, the defects such as voids are generated in the connecting portion 13'. Particularly, since a water component or a gas component included in an outer surface of the sheath 3', to which a center portion 10' of the shed 5' is faced, has a long way for degassing, there is an extremely high possibility such that a water component or a gas component is closed in the connection portion.
On the other hand, in the method (2) in which the connection is performed by using the adhesives, since the securing force at the connecting portion is uniform as is the same as the method (1), a moving direction of the adhesives is not fixed in the case of a connecting operation. Therefore, there is an extremely high possibility such that a water component or the like included in the adhesives is closed in the connecting portion.
In order to eliminate the residual water component or gas component in the connecting portion, the inventors further discussed the methods (1) and (2) mentioned above. As a result, the inventors found it preferable that, in both methods, a flow of a water component or a gas component in the non-cured rubber and a movement of the adhesives are controlled in a predetermined direction by adjusting the securing force of the shed with respect to the sheath.